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    <font size="7">Filesystem Version 3<br>
    Design</font></td>
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      <i><b>Contents</b></i></td>
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    <td width="100%" bgcolor="#E8F5FF">
    <a href="#Introduction">Introduction</a><br>
    <a href="#Problem">Problem</a><br>
    <a href="#Solution">Solution</a><br>
    <a href="#Details">Details</a><br>
    <a href="#Other-changes">Other changes</a><br>
    <a href="#Acknowledgements">Acknowledgements</a></td>
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<p><b>Caution:</b> This page documents thinking early in the V3 development
process, and is intended to serve historical purposes. It is not updated to
reflect the current state of the library.</p>

<h2><a name="Introduction">Introduction</a></h2>

<p>During the review of Boost.Filesystem.V2 (Internationalization), Peter Dimov
suggested that the <code>basic_path</code> class template was unwieldy, and that a single
path type that accommodated multiple character types and encodings would be more
flexible. Although I wasn't willing to stop development at that time to
explore how this idea might be implemented, or to break from the pattern for
Internationalization used the C++ standard library, I've often thought about
Peter's suggestion. With the advent of C++0x <code>char16_t</code> and <code>char32_t</code>
character
types, the <code>basic_path</code> class template approach becomes even more unwieldy, so it
is time to revisit the problem in light of Peter's suggestion.</p>

<h2><b><a name="Problem">Problem</a></b></h2>

<p>With Filesystem.V2, a path argument to a user defined function that is to
accommodate multiple character types and encodings must be written as a
template. Do-the-right-thing overloads or template metaprogramming must be
employed to allow arguments to be written as string literals. Here's what it
looks like:</p>

<blockquote>
  <pre>template&lt;class Path&gt;
void foo( const Path &amp; p );</pre>
  <pre>inline void foo( const path &amp; p )
{
  return foo&lt;path&gt;( p );
}
inline void foo( const wpath &amp; p )
{
  return foo&lt;wpath&gt;( p );
}</pre>
</blockquote>
<p>That's really ugly for such a simple need, and there would be a combinatorial
explosion if the function took multiple Path arguments and each could be either
narrow or wide. It gets even worse if the C++0x <code>char16_t</code> and <code>
char32_t</code> types are to be supported.</p>

<h2><a name="Solution">Solution</a></h2>

<p>Overview:</p>

<ul>
  <li>A single, non-template, <code>class path</code>.</li>
  <li>Each member function is a template accommodating the various
  applicable character types, including user-defined character types.</li>
  <li>Hold the path internally in a string of the type used by the operating
  system API; <code>std::string</code> for POSIX, <code>std::wstring</code> for Windows.</li>
</ul>

<p>The signatures presented in <a href="#Problem">Problem</a> collapse to
simply:</p>
<blockquote>
  <pre>void foo( const path &amp; p );</pre>
</blockquote>

<p>That's a signification reduction in code complexity. Specification becomes
simpler, too. I believe it will be far easier to teach, and result in much more
flexible user code.</p>

<p>Other benefits:</p>
<ul>
  <li>All the polymorphism still occurs at compile time.</li>
  <li>Efficiency is increased, in that conversions of the encoding, if required,
  only occur once at the time of creation, not each time the path is used.</li>
  <li>The size of the implementation code drops approximately in half and
  becomes much more readable.</li>
</ul>
  <p>Possible problems:</p>
<ul>
  <li>The combination of member function templates and implicit constructors can
  result in unclear error messages when the user makes simple commonplace coding
  errors. This should be much less of a problem with C++ concepts, but in the
  meantime work continues to restrict over aggressive templates via enable_if/disable_if.</li>
</ul>
  <h2><a name="Details">Details</a></h2>

<table border="1" cellpadding="4" cellspacing="0" style="border-collapse: collapse"
 bordercolor="#111111" width="100%">
  <tr>
    <td width="33%" colspan="3">
    <p align="center"><b><i>Encoding </i></b><i><b>Conversions</b></i></td>
  </tr>
  <tr>
    <td width="33%">
    <p align="center"><i><b>Host system</b></i></td>
    <td width="33%">
    <p align="center"><i><b>char string path arguments</b></i></td>
    <td width="34%">
    <p align="center"><i><b>wide string path arguments</b></i></td>
  </tr>
  <tr>
    <td width="33%">Systems with <code>char</code> as the native API path character type (i.e.
    POSIX-like systems)</td>
    <td width="33%">No conversion.</td>
    <td width="34%">Conversion occurs, performed by the current path locale's
    <code>codecvt</code> facet.</td>
  </tr>
  <tr>
    <td width="33%">Systems with <code>wchar_t</code> as the native API path character type
    (i.e. Windows-like systems).</td>
    <td width="33%">Conversion occurs, performed by the current path locale's
    <code>codecvt</code> facet.</td>
    <td width="34%">No conversion.</td>
  </tr>
</table>

<p>When a class path function argument type matches the operating system's
API argument type for paths, no conversion is performed rather than conversion
to a specified encoding such as one of the Unicode encodings. This avoids
unintended consequences, etc.</p>

<h2><a name="Other-changes">Other changes</a></h2>

<p><b>Uniform hybrid error handling: </b>The hybrid error handling idiom has
been consistently applied to all applicable functions.</p>

<h2><a name="Acknowledgements">Acknowledgements</a></h2>

<p>Peter Dimov suggested the idea of a single path class that could cope with
multiple character types and encodings. Walter Landry contributed both the design
and implementation of the copy_any,
copy_directory, copy_symlink, and read_symlink functions.</p>

<hr>

<p>&copy; Copyright Beman Dawes, 2008</p>
<p> Use, modification, and distribution are subject to the Boost Software
License, Version 1.0. See <a href="http://www.boost.org/LICENSE_1_0.txt">
www.boost.org/LICENSE_1_0.txt</a></p>

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